As hydrocarbon-producing wells mature, water production becomes a serious problem. Remediation techniques for controlling water production are generally referred to as conformance control. Remediation techniques are selected on the basis of the water source and the method of entry into the wellbore. Conformance control treatments include sealant treatments and relative permeability modifiers (also referred to as disproportionate permeability modifiers).
In previous years, water-soluble chromium (+3) crosslinked polyacrylamide gels have been used in conformance control treatments. The gel time was usually controlled by the addition of materials that chelate with chromium in competition with the polymer-bound carboxylate groups. The crosslinking reactions in these gel systems take place by the complexation of Cr (+3) ions with carboxylate groups on the polymer chains. Because of the nature of the chemical bond between Cr (+3) and the pendant carboxylate groups, formation of insoluble chromium species can occur at high pH values. Other problems with these systems include thermal instability, unpredictable gel time, and gel instability in the presence of chemical species that are potential ligands.
Another water-based gel system for conformance control is based on phenol/formaldehyde crosslinker system for homo-, co-, and ter-polymer systems containing acrylamide. The crosslinking mechanism involves hydroxymethylation of the amide nitrogen, with the subsequent propagation of crosslinking by multiple alkylation on the phenolic ring. Because of the nature of this chemical bond, the gel time is controllable over a wide temperature range. Although these gels work well, phenol and formaldehyde are highly toxic.
U.S. Pat. No. 5,836,392 discloses a system based on a polyethyleneimine (PEI) crosslinker and a copolymer of acrylamide and t-butyl acrylate (PA-t-BA). PEI is such a low-toxicity material that the Food and Drug Administration has approved it in the United States for food contact. Although non-toxic, PEI can bioaccumulate or persist in the environment for long periods.
It has been shown that chitosan can be used in place of polyethyleneimine to crosslink acrylamide based polymers. Chitosan's usefulness as a crosslinker has been limited, however, by its relative poor solubility in aqueous solutions. For example, commercial sources of chitosan are only sparingly soluble in water; about 1–2% active solutions are the highest concentrations that can be made while maintaining usable viscosity. While this is a step forward in the effort to provide more environmentally acceptable systems, the major component, or base polymer, of such a gel system is still a non-biodegradable polymer. The major component of such gel system is generally a homopolymer or copolymer of acrylate-type monomers, such as acrylic acid, acrylamide, vinylpyrrolidone etc. The backbone of such polymers contains continuous carbon-carbon single bonds, which are of poor biodegradability. Since the chitosan crosslinker is only a minor component of the gel composition, the total system is still predominantly non-biodegradable due to poor biodegradability of the synthetic base polymer.